Because the pseudo-random number generation may be WAY faster than true random number source. Like "delay between keypresses", 1s or so, or if you pick the noise one bit a time, from some slow source - say, your ADC takes 30 cycles for one conversion and has only 2 bits of usable noise, for 32-bit seed you'd need 16 conversions. Many good sources of enthropy are slow, in this case they are useful for single seeding of the generator, and then you use fast pseudorandom RNG to generate the following random numbers.

By the way, has anyone here tried combining key debounce procedure with picking entropy?